In known isocyanate production facilities, a phosgene solution and an amine solution are introduced into a reactor where the phosgene and amine react to form an isocyanate. The reaction products are generally separated in a distillation column to obtain purified isocyanate and often further purified in a distillation or crystallization unit to separate its isomers. In order to ensure production of a predetermined amount of the isocyanate, the amount of phosgene and of amine introduced into the reactor and/or column is controlled. Further, to ensure production of isocyanate satisfying predetermined quality requirements, the process parameters of the reactor such as pressure and temperature are controlled. The distillation column used to separate the materials in the reacted reaction mixture is designed so that the isocyanate separated in that column will be obtained in the required amount and quality from the reacted reaction mixture. It is possible to calculate the optimal process parameters for the isocyanate production process in the steady state. These calculated process parameters are taken as controlled variables to automatically keep them relatively constant in case of an unsteady disturbance. Generally, it is possible to control every controlled variable independently of the others. When, for example, the temperature of the distillation column decreases due to cold weather, the amount of heat for the distillation column is increased to keep the optimized reference temperature of the distillation column constant.
It is preferred to feed excess phosgene to the reactor in order to influence the chemical equilibrium of the reactor contents and to maximize the isocyanate output. Since phosgene is a highly toxic and harmful gas, the excess phosgene must be eliminated after the reaction is complete. Alternatively, the excess phosgene may be recycled to the reactor and thereby increase cost effectiveness. The solvent obtained in the distillation step can also be recycled. However, use of recycled phosgene and solvent may cause the isocyanate production process to become unstable. Since unavoidable variations of the process parameters affect the amount, the pressure, the temperature, the concentration (quality) etc. of the recycled phosgene and solvent, variations in the process parameters and disturbances may even increase when automatic control of the process parameters is attempted. Due to the recycling, nearly every process parameter influences nearly all other process parameters. Thus, it is necessary to manually set a number of reference parameters based on the overall situation to avoid major variation of the amount and quality of the product isocyanate. Due to the potential for large variation of the isocyanate product quality, conservative reference parameters are set for the isocyanate production process to ensure a minimum quality of the isocyanate. This results in a difficult and complicated process control and high costs.
It is an objective of the present invention to facilitate the process control of a production process, particularly, an isocyanate production process. It is a further objective to reduce variations in the product, e.g., isocyanate, to increase the product output, and to reduce the production costs. Furthermore, it is an objective to improve and/or increase automation of the production process. It is another objective of the present invention to increase the stability of the automated process control while the amount of unused reactant, e.g., phosgene, and spent solvent is preferably reduced. Moreover, the amount of unwanted substances in the production process should be reduced.